Reproductive aging is characterized by changes in estrogen levels; however, the brain itself is subject to age-related changes, some of which may be consequences of altered estrogen levels, and others which may be estrogen-independent. Therefore, the present proposal will examine the interactions of estrogen with hypothalamic neurons in the brain during reproductive senescence. Studies will focus on estrogen regulation of gonadotropin-releasing hormone (GnRH) neurons, the key cells regulating the reproductive axis, as well as the mediation of estrogen's effects on GnRH neurons by glutamatergic neurons, acting via the NMDA receptor. In order to better characterize effects of estrogen and hypothalamic aging, as well as their interactions, studies will be performed in the experimental model of ovariectomy (OVX), with or without estrogen replacement, as well as the natural model of ovarian- intact animals, at different stages of reproductive function and endogenous estrogen levels. Specific Aims 1 and 2 will investigate mechanisms for estrogen's effects on hypothalamic function during aging in young, middle-aged (MA) and old Sprague-Dawley OVX rats, given long-term or short-term estrogen or vehicle replacement (Aim 1) or in ovarian-intact young, MA and old rats during the estrous cycle (Aim 2). For Aim 2, animals are utilized on pro-estrus the day of the pre-ovulatory GnRH/LH surge, with high estrogen levels, or on diestrus I, a day of low estrogen levels. Acyclic rats will be utilized on persistent estrus (high estrogen) or persistent estrus (high estrogen) or persistent diestrus (low estrogen). In both of these aims, effects of estrogen on GnRH and NMDA receptor subunit mRNAs and proteins will be quantitated by RNase protection assay and Western blots, respectively. The localization of NMDA receptor subunits on GnRH neurons, and their regulation by estrogen and aging, will be determined. The responsiveness of the GnRH neurosecretory system to administration of an NMDA receptor agonist will also be assessed. Specific Aims 3 and 4 will investigate the physiological changes in estrogen regulation of hypothalamic neurons, using the rhesus monkey, with its 28 day menstrual cycle, as an experimental model. Aim 3 will examine changes in hypothalamic GnRH and glutamate release and the regulation of GnRH release by NMDA receptor activation in OVX young and old monkeys, with or without long-term or short-term estrogen replacement. Aim 4 will make similar measurements in ovarian-intact young and old rhesus monkeys across the natural menstrual cycle during periods of high estrogen (late follicular phase) or low estrogen (late luteal phase); comparisons will be made with amenorrheic aged monkeys. Experiments are anticipated to provide insight into the neurobiological effects of an estrogen deficiency at menopause.